Varicella vaccine, also known as chickenpox vaccine, is a vaccine that protects against chickenpox.[1] One dose of vaccine prevents 95% of moderate disease and 100% of severe disease.[2] Two doses of vaccine are more effective than one.[2] If given to those who are not immune within five days of exposure to chickenpox it prevents most cases of disease.[2] Vaccinating a large portion of the population also protects those who are not vaccinated.[2] It is given by injection just under the skin.[2]

The World Health Organization (WHO) recommends routine vaccination only if a country can keep more than 80% of people vaccinated.[2] If only 20% to 80% of people are vaccinated it is possible that more people will get the disease at an older age and outcomes overall may worsen.[2] Either one or two doses of the vaccine is recommended.[2] In the United States two doses are recommended starting at twelve to fifteen months of age.[1] As of 2012 most European countries either recommend it for all children or just those at high risk,[3] but not all countries provide the vaccine due to its cost.[4]

The vaccine is very safe.[1] Minor side effects may include pain at the site of injection, fever, and rash.[1] Severe side effects are rare and occur mostly in those with poor immune function.[2] Its use in people with HIV/AIDS should be done with care.[2] It is not recommended during pregnancy; however, the few times it has been given during pregnancy no problems resulted.[1][2] The vaccine is available either by itself or along with the MMR vaccine, in a version known as the MMRV vaccine.[2] It is made from weakened virus.[1]

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Varicella vaccine is 70% to 90% effective for preventing varicella and more than 95% effective for preventing severe varicella.[7] Follow-up evaluations have taken place in the United States of children immunized that revealed protection for at least 11 years. Also, studies were conducted in Japan which indicated protection for at least 20 years.[7]

People who do not develop enough protection when they get the vaccine may develop a mild case of the disease when in close contact with a person with chickenpox. In these cases, people show very little sign of illness. This has been the case of children who get the vaccine in their early childhood and later have contact with children with chickenpox. Some of these children may develop a mild chickenpox also known as breakthrough disease.[8]

Another vaccine, known as zoster vaccine, is simply a larger-than-normal dose of the same vaccine used against chickenpox, and is used in older adults to reduce the risk of shingles (also called herpes zoster) and postherpetic neuralgia, which are caused by the same virus.

The long-term duration of protection from varicella vaccine is unknown, but there are now persons vaccinated twenty years ago with no evidence of waning immunity, while others have become vulnerable in as few as six years. Assessments of duration of immunity are complicated in an environment where natural disease is still common, which typically leads to an overestimation of effectiveness.[9]

Some vaccinated children have been found to lose their protective antibody in as little as five to eight years.[10] However, according to the World Health Organization: "After observation of study populations for periods of up to 20 years in Japan and 10 years in the United States, more than 90% of immunocompetent persons who were vaccinated as children were still protected from varicella." However, since only one out of five Japanese children were vaccinated, the annual exposure of these vaccinees to children with natural chickenpox boosted the vaccinees' immune system. In the United States, where universal varicella vaccination has been practiced, the majority of children no longer receive exogenous (outside) boosting, thus, their cell-mediated immunity to VZV (varicella zoster virus) wanes—necessitating booster chickenpox vaccinations.[11] As time goes on, boosters may be necessary. Persons exposed to the virus after vaccine tend to experience milder cases of chickenpox.[12]

Catching "wild" chickenpox as a child has been thought to commonly result in lifelong immunity. Indeed, parents have deliberately ensured this in the past with "pox parties". Historically, exposure of adults to contagious children has boosted their immunity, reducing the risk of shingles.[13] The CDC and corresponding national organisations are carefully observing the failure rate which may be high compared with other modern vaccines—large outbreaks of chickenpox having occurred at schools which required their children to be vaccinated.[14][15][16][17]

Prior to the introduction of the vaccine in 1995 in the United States (released in 1988 in Japan and Korea), there were around 4,000,000 cases per year in the United States, mostly children, with typically 10,500–13,000 hospital admissions (range, 8,000–18,000), and 100–150 deaths each year.[11][18] Though mostly children caught it, the majority of deaths (by as much as 80%) were among adults.

During 2003 and the first half of 2004, the US Centers for Disease Control and Prevention (CDC) reported eight deaths from varicella, six of whom were children or adolescents. These deaths and hospital admissions have substantially declined in the US due to vaccination,[19][20] though the rate of shingles infection has increased as adults are less exposed to infected children (which would otherwise help protect against shingles).[21][22][23] Ten years after the vaccine was recommended in the US, the CDC reported as much as a 90% drop in chickenpox cases, a varicella-related hospital admission decline of 71%[18] and a 97% drop in chickenpox deaths among those under 20.[24]

Vaccines are less effective among high-risk patients, as well as being more dangerous because they contain attenuated live virus. In a study performed on children with an impaired immune system, 30% had lost the antibody after five years, and 8% had already caught wild chickenpox in that five-year period.[25]

Herpes zoster (shingles) most often occurs in the elderly and is only rarely seen in children. The incidence of herpes zoster in vaccinated adults is 0.9/1000 person-years, and is 0.33/1000 person-years in vaccinated children; this is lower than the overall incidence of 3.2–4.2/1000 person-years.[26][27]

Adult shingles cases may increase after introduction of varicella vaccine, but evidence is unclear.[13][27][28][29] While research using computer models has tended to support the hypothesis that vaccination programs would increase incidence of zoster in the short term, the evidence from epidemiological studies is mixed,[30][31] and increases observed in zoster incidence in some studies may not be related to vaccination programs, as the incidence increases prior to the varicella vaccine program being initiated.[32]

Regarding herpes zoster, the US Centers for Disease Control states: "Chickenpox vaccines contain weakened live VZV, which may cause latent (dormant) infection. The vaccine-strain VZV can reactivate later in life and cause shingles. However, the risk of getting shingles from vaccine-strain VZV after chickenpox vaccination is much lower than getting shingles after natural infection with wild-type VZV."[33]

The World Health Organization recommends one or two doses with the initial dose given at 12 to 18 months of age.[2] The second dose if given should occur at least one to three months later.[2] This vaccine is a shot given subcutaneously (under the skin). It is recommended for all children under 13 and for everyone 13 or older who has never had chickenpox.

In the United States two doses are recommended. In the first case a first dose is administered at 12 to 15 months and a second dose at age 4–6 years. For people older than 13 the two doses are administered 4 to 8 weeks apart.[34]. The varicella vaccine did not become widely available in the United States until 1995 [35].

In the UK, the vaccine is only recommended in people who are particularly vulnerable to chickenpox.[36]

The varicella vaccine is not recommended for seriously ill people, pregnant women, people who have experienced a serious allergic reaction to the varicella vaccine in the past, people who are allergic to gelatin, people allergic to neomycin, people receiving high doses of steroids, people receiving treatment for cancer with x-rays or chemotherapy, as well as people who have received blood products or transfusions during the past five months.[37] It may be usable in people with HIV infections who have a good blood count and are receiving appropriate treatment.[2]

Serious side effects are very rare. From 1998 to 2013, only one vaccine-related death was reported: an English child with pre-existent leukemia. In some occasions, severe reactions have been reported such as meningitis and pneumonia (mainly in inadvertently vaccinated immunocompromised children) as well as anaphylaxis.[26]

The possible mild side effects include redness, stiffness, and soreness at the injection site, as well as fever. A few people may develop a mild rash, which usually appears around the injection site.[38]

There is a short-term risk of developing herpes zoster (shingles) following vaccination. However, this risk is less than the risk due to a natural infection resulting in chickenpox.[39]:378 Most of the cases reported have been mild and have not been associated with serious complications.[40]

Approximately 5% of children who receive the vaccine develop a fever or rash. Adverse reaction reports for the period 1995 to 2005 found no deaths attributed to the vaccine despite approximately 55.7 million doses being delivered.[41] Cases of vaccine-related chickenpox have been reported in patients with a weakened immune system,[42][43] but no deaths.

The literature contains several reports of adverse reactions following varicella vaccination, including vaccine-strain zoster in children and adults.[44] A mean of 2,350 reports per year of an adverse event the occurs some time after varicella vaccine is administered, based on 20,004 cases reported to the Vaccine Adverse Event Reporting System (VAERS) database from May 1995 through December 2003; however, this includes all adverse events, whether or not caused by the vaccine and some events are to be statistically expected regardless (independent of any vaccine administration). VAERS reports do not imply any finding of causation. Minor events are known to be underreported to VAERS.

One study has indicated that VZV vaccine protects against oral and genital herpes outbreaks.[45]

The Varicella zoster vaccine is made from the Oka/Merck strain of live attenuated varicella virus. The virus was initially obtained from a child with natural varicella, introduced into human embryonic lung cell cultures, adapted to and propagated in embryonic guinea pig cell cultures, and finally propagated in human diploid cell cultures.[46]

Japan was among the first countries to vaccinate for chickenpox. Routine vaccination against varicella zoster virus is also performed in the United States, and the incidence of chickenpox has been dramatically reduced there (from 4 million cases per year in the pre-vaccine era to approximately 400,000 cases per year as of 2005). In Europe most countries do not currently vaccinate against varicella, though the vaccine is gaining wider acceptance. Australia, Canada, and other countries have now adopted recommendations for routine immunization of children and susceptible adults against chickenpox.

Other countries, such as the United Kingdom, have targeted recommendations for the vaccine, e.g., for susceptible health care workers at risk of varicella exposure. In the UK, varicella antibodies are measured as part of the routine of prenatal care, and by 2005 all National Health Service personnel had determined their immunity and been immunized if they were non-immune and have direct patient contact. Population-based immunization against varicella is not otherwise practised in the UK.

A controversy has arisen, in part, over concern due to the role of human embryonic cell lines and vaccine development. Unlike bacteria or fungi, viruses cannot reproduce autonomously and require a living organism host in order to replicate. The ability of the virus to cause infection usually depends on the host with whom its cells have adapted to over generations of replication. In other words, a virus can lead to a more severe infection in certain organisms versus others. To create a vaccine that is safe for individuals while having the ability to induce a protective immune response necessitates choosing an appropriate cell line to grow a virus.

WI-38 and MRC-5 are embryonic lung cell lines derived nearly 35 years ago from two post-mortem fetuses. They have been maintained in laboratory conditions since, self-replicating and producing more embryonic lung cells that can be used for research and other scientific purposes. These cells cannot develop into a human organism and have no potential for human life. Furthermore, no additional tissue from post-mortem fetuses is required for their existence.

One of the uses of these cell lines is to provide an environment for virus replication. The Oka/Merck strain of live attenuated varicella virus is one of the viruses that require this environment to grow.[47] Some controversy has arisen due to the ideas that the vaccine contains components of or has required the use of an aborted fetus, which violates the ethics and beliefs of many who oppose the use of aborted fetuses in medical research. While the cell line provides the environment in which the virus is grown and subsequently isolated from, theoretically, the vaccine itself is free from extraneous biological agents.[48] Nevertheless, Merck & Co., Inc.'s M-M-R®II, ZOSTAVAX® and VARIVAX® vaccine descriptions state that "residual components of MRC-5 cells including DNA and protein" are present in the vaccine.[49][50][51] Also, the use of the WI-38 and MRC-5 cell lines for growing the varicella virus is controversial because these human embryonic lung cells were derived from aborted fetuses. Due to beliefs concerning the use of human embryonic fetal tissue in medical research, some individuals are opposed to using the varicella vaccine.[47]